British Gas is set to hit millions of families with a rise in energy prices that could add £100 a year to household bills.

The country’s biggest power supplier is preparing to reveal as early as tomorrow that it is putting up its gas and electricity tariffs to record levels, the Standard has learned.

Its rise is expected to be in “high single digits” — close to the nine per cent increase from rival Scottish & Southern Energy that comes into force on Monday.

The average British Gas dual fuel bill for gas and electricity now stands at £1,260. A five per cent rise would add £63, while an eight per cent increase would put on £100. The new higher prices are expected to kick in next month.

The supplier, which has about 16 million domestic customers, will say it is forced into the move by rising wholesale energy and other costs. However, consumer groups insisted its huge profits mean it could easily afford to delay the rise.

Parent company Centrica’s first-half profit was up 15 per cent to £1.45 billion. That included £345 million, a 23 per cent rise, from supplying power and heating to homes.

The planned move will raise new fears for the growing number of Britons in fuel poverty — those who spend more than 10 per cent of their income on gas and electricity.

Ann Robinson, director of consumer policy at uSwitch, said: “This announcement will send consumers reeling. It is the death knell for any hope that suppliers would hold off from price rises until after winter. Consumers will be bitterly disappointed. The pressure of an extra £100 or so on energy bills will leave many buckling and forced to face another winter when they are scared to turn on heating for fear of cost.”

Last year, when all the big six power suppliers increased their prices, eight in 10 British households rationed their energy use because of cost worries.

Shadow energy secretary Caroline Flint said: “People will not understand why British Gas are putting up prices after their interim financial report showed profits on UK residential customers up 23 per cent compared with the same period last year.”

British Gas warned as early as May that bills would have to go up. From Monday average dual bills for the five million customers of Britain’s second-biggest energy company SSE, which operates in London as Southern Electric, will hit £1,354 a year.

Other big suppliers — including EdF, Scottish Power and NPower — are now expected to announce higher prices. E.ON, the only supplier to guarantee a price freeze for 2012, this week refused to extend this commitment into 2013.

All suppliers cut their prices slightly at the start of the year, with British Gas lowering its electricity tariff by five per cent. Although wholesale energy costs have remained well below their peaks this year, the prices which British Gas has had to pay for gas in advance of the winter are about 15 per cent higher than last year.

Energy Secretary Ed Davey said: “The fact is that no government can control world energy prices — but we are acting to help people cut the size of the bills they actually pay.

“That’s why we encourage people to switch to better deals and we are looking at new ways to make that easier, like group purchase.

“It’s also why we want to help people take advantage of insulation offers, so their homes are warmer for less, and why we have special help with energy bills for those most in need.”

Shadow energy secretary Caroline Flint said: “People will not understand why British Gas are putting prices up, despite their interim financial report showing profits on UK residential customers being up by 23 per cent, compared to the same period last year.

“Hard-pressed families and businesses need much more transparency on costs, pricing and profits to know whether they’re getting a fair deal, but the Government’s Energy Bill does nothing to reform the energy market. We need a one nation government to break the dominance of the energy giants, to simplify tariffs, and to protect vulnerable customers from being ripped off.

“Unless ministers get to grips with spiralling energy bills, people will rightly think that this Government is completely out of touch with families and pensioners struggling to make ends meet.”

British Gas said: “We do not comment on future pricing decisions.”

Nights draw in and bills rise

Analysis: Jonathan Prynn Consumer Business Editor

IT’S THAT time of year again. Nights drawing in, clocks soon to go back, central heating on and, bang, up go energy bills. First Southern Electric with its nine per cent rise on Monday with British Gas and the others following.

The industry knows that its credibility is on the line. Labour leader Ed Miliband has placed the rising cost of those essentials — heat, light and power — right at the top of the political agenda.

The “big six” suppliers have promised to clean up their act by making bills easier to understand and that is welcome. But at a time when there are ominous rumblings about rising food bills, it will come as grim news for most families that another of their unavoidable costs will be going up as Christmas approaches.

The suppliers argue that they have to make respectable profits to fund the vast investment needed in new power stations and energy infrastructure. And there is much truth in that. Their problem is that a legacy of doorstep mis-selling, poor customer service and indecipherable billing means that trust in energy companies is little higher than in banking.

They are working hard to put that right and it’s true that many of the costs they face are beyond their control, some of them imposed by government. The new “face” of trade body Energy UK — Angela Knight — likes a challenge: she previously led the British Bankers’ Association. It’s going to be a tough gig.